In digital communications systems equalisers are used to compensate for multipath delay where the same signal is propagated over a number of different paths. In simulcast systems, such as Motorola's ASTRO/APCO simulcast systems, the same signal information is transmitted from multiple transmitters to increase coverage.
In a digital transmission consisting of a series of frames, each frame containing a number of known training symbols followed by a number of data symbols, a training sequence is used to initialise the settings of the equaliser. The correct settings depend on the state of the multipath channel and vary in time due to fading. An equaliser must continue to adjust its settings during the data block if the channel changes are significant during the frame. Such equalisers are referred to as adaptive. In TDMA systems such as USDC and TETRA, the number of data symbols over which the equaliser must adapt is of the order of 100. In ASTRO, the number is over 800.
For an adaptive equaliser, the bit error rate tends to increase as the frame size increases. This is because of error propagation. If a bit error occurs, the adaptive algorithm may diverge from its proper state giving bit errors for the rest of the frame. Thus, higher bit error rates occur for adaptive equalisers applied to systems such as ASTRO than for systems with more regular training sequences. In systems such as ASTRO simulcast systems, it is desirable to have an adaptive equaliser so that higher propagation delays between signals arriving from different transmitters may be tolerated, thus, increasing simulcast site separations while providing acceptable bit error rate performance.
Furthermore, it is desirable to allow the design of systems with fewer training sequences for equalisation which gives the benefit of reduced overhead and hence more capacity for user data.